Requirements for infrared domes and similar applications include hardness, infrared transparency, fracture toughness, and thermal shock resistance. There are currently three commercially available materials for use in applications such as infrared domes which can transmit in the infrared region of about 3-5 microns: ZnS, spinel, and sapphire. Although these materials are excellent in some respects, they all have important limitations.
ZnS has excellent transparency and is readily formed into the specific shapes required for windows and domes. However, this material is very soft and is abraded under normal operating conditions. The abrasion limits both the useful scope and the lifetime of products made from ZnS. Some improvement of the abrasion resistance is possible through the application of protective coatings, but the high temperatures which are rapidly generated during high speed acceleration leads to delamination. For instance, a speed of Mach 4, or about 2800 mph, generates a temperature of about 600.degree. C.
Spinel has a slightly diminished transparency as compared to ZnS, i.e., the 5-micron radiation is partially absorbed. Since this material is about ten times harder than ZnS, it has greater erosion resistance. It is also better at resisting thermal shock because of higher fracture toughness than ZnS. However, difficulty in the fabrication of required shapes makes windows and domes of this material slightly more expensive than those made from ZnS.
Sapphire has a transparency about equal to that of spinel and has hardness and fracture toughness properties that are about 40% greater than those of spinel. Thus, windows or domes produced from sapphire are highly abrasion resistant and can be used for supersonic applications. However, this material is very expensive to produce and even more expensive to fabricate into the required configurations.